If both the versions 4x4 and 4x2 share the same body and chassis, then you might have some luck. However, if not, you will have very little chance. It would be cheaper, and less of a nightmare, just to trade in your 4x2 for a 4x4.
First of all you need to install a transfer case, a front propellor shaft, a front differential and axles. The front suspension may need to be replaced to handle this extra hardware. You would need to make sure that the engine will clear the front axles.
You would have to change the rear prop shaft, or shorten it, to compensate for the transfer case. You will probably need to change your gearbox as well, because 4x2 gearboxes usually don't have the mounting points to attach a transfer case to. If the new gearbox doesn't mate up to the engine, you would need to make up an adapter plate to join them.
You would need to fabricate (i.e. cut) a hole in the floor somewhere for the 4WD lever to go.
Best of Luck.